Polymerization of vinyl monomers to prepare polyvinyl resins has been extensively research and practiced. Economic competition in the thermoplastic manufacturing industry has created a marketplace in which the consumer enjoys a plethora of plastic products, however, there exists an ever increasing demand for lower priced, quality products. The widespread commercial success of thermoplastic products prepared from polyvinyl resins has generated incentive to search for more efficient methods of preparation.
In order to improve commercial preparation of plastic products, efforts have been made to improve the polymerization processes which provide the feedstock polyvinyl resin for further processing. Various initiators and catalysts for polymerization have been taught.
Howard, U.S. Pat. No. 2,610,293 discloses hydrazone and peroxide catatlysis of the polymerization of vinyl monomers.
Van Peski, U.S. Pat. No. 2,478,066 discloses hydrazone as a polymerizatio promoter for vinyl monomers, the reaction taking place at high temperature under pressurized conditions.
Conventional polymerization processes require relatively high temperatures to first initiate polymerization and then to provide for high rates of conversion of the monomeric unsaturate to the polymeric resin. Elevated temperatures degrade the resin thus adversely effecting the structural properties of the thermoplastic product.
In order to polymerize vinyl monomers at lower temperatures, azo and peroxide initiators have been utilized alone and in combination with other catalysts. A disadvantage to the commercial use of reactive chemicals such as peroxides and azo compounds is that they are hazardous to use and production costs increase accordingly. It would be highly desirable to be able to polymerize vinyl monomers under milder, safer and less costly reaction conditions.
There also exists a need in the industry to find more efficient methods for preparation of cross-linked and branched vinyl polymers. Branched vinyl polymers, sometimes referred to as starbranched polymers, are characterized by a number of linear polymeric chains joined at a central point. Long chain branching decreases the molecular dimensions of a branched polymer compared to the molecular dimensions of a linear polymer of the same molecular weight (MW). The more compact branched polymer is less viscous compared to the corresponding linear polymer. The less viscous branched polymers can be processed at desirably high rates at lower pressures and temperatures than the corresponding linear polymers of the same MW.
Molten branched polymeric resins have a higher melt flow rate than molten linear polymeric resins of comparable MW. Therefore, branch polymers are also desirably used for injection molding applications. The high flow rate at lower resin temperatures results in a shorter mold cycle time.
High MW branched polymers (greater than 100,000 MW) are more easily processed than high MW linear polymers due to decreased viscosity at a given MW. High MW polymers are used to prepare plastic products with a desirable amount of structural integrity, such as impact-resistance, tensile strength and toughness.
Many branched polymers are prepared using processes of polymerization which employ multi-sited anionic or cationic initiators. These processes are undesirably costly in that highly purified reactants are prerequisite and stringent polymerization conditions require the essential absence of oxygen and water vapor.
It would be desirable to have multi-sited initiators for free-radical polymerization processes which are carried out under less stringent and hence less costly conditions than are ionic polymerization processes.
Crivello et al., Polym. Bull. 16(2--3), 95--102 (1986) discloses a cyclic silyl pinocle ether for use in free-radical polymerization of vinyl monomers.